As is well known to those skilled in the art, processing of crude oil may include many steps in order to permit ultimate recovery of maximum quantity of products boiling in the gasoline and middle distillate boiling range. Typically crude oil is subjected to atmospheric distillation to yield overhead including light ends and fractions boiling in the gasoline boiling range, intermediate side cuts typified by gas oils, and as bottoms a topped or reduced crude. The latter is commonly subjected to further distillation in a vacuum tower from which are commonly recovered inter alia a vacuum gas oil fraction and as bottoms vacuum resid. The gas oil cuts are commonly passed to catalytic cracking wherein they are converted to fractions boiling in the gasoline and middle distillate boiling range.
The crudes which are used with increasing frequency are those with a higher content of undesirable components, particularly sulfur. It is not uncommon to charge crude containing as much as 2 wt % sulfur. This sulfur content is found in the products of atmospheric distillation including the topped crude. The vacuum gas oil, recovered from vacuum distillation, may be found to contain as much as 4 wt % sulfur. If this vacuum gas oil be passed directly to catalytic cracking, as has heretofore been done in the case of streams derived from low sulfur crudes, the sulfur undesirably shows up in the regenerator off-gas from the catalytic cracking unit. The presence of sulfur oxides in regenerator off-gas is undesirable because emission standards are becoming increasingly strict; and there is no way to economically remove sulfur oxides from the regenerator off-gas. Because of this, it has been increasingly common to attempt to remove sulfur from various streams that serve as feedstock for the catalytic cracking unit.
One approach to this problem is to subject the virgin gas oil to solvent extraction prior to admission to catalytic cracking This may be carried out for example in an MP refining operation in which the charge gas oil is contacted with N-methyl pyrrolidone. This treatment removes sulfur and nitrogen components from the gas oil; and it also removes aromatics. In typical operation, MP refining may reduce the sulfur content of a gas oil (with an initial sulfur content of 2 w %) down to as low as 0.7 w %. The nitrogen content may also be reduced from an initial level of 750 wppm down to a final level of 250. Simultaneously the aromatics content may be reduced from an initial level of 50 w % to a final value of 30 w %.
MP refining may yield a solvent refined oil of reduced volume. Typically such an oil may be only 70-80 v % of that originally charged to the MP unit; and the decrease will reflect the volume of aromatics which have been extracted from the oil. It is not uncommon to find that MP refining of a virgin gas oil may yield a solvent extracted gas oil which is as little as only 70 w % of the volume of virgin vacuum gas oil charged to solvent refining This means that the catalytic cracking operation to which the refined gas oil is passed will operate substantially below design capacity based on untreated vacuum gas oil.
It is an object of this invention to provide a process for operating a petroleum refinery at increased efficiency. Other objects will be apparent to those skilled in the art.